I found a little how-to over at OSXFAQ on how to do a proper static route on OS X Leopard, which will persist through reboots. Please, go over there and read it, if you'd like. I've got a couple minor changes to make it a bit clearer as to what it does:

To add a static route you need to issue a command like:

sudo route -nv add -net 192.168/16 -interface en0

To avoid having to do this everytime you reboot your system, we need to build a custom startup script. The Apple/OS X way to do this, is to put a script in /System/Library/StartupItems. Let's build the framework first:
<pre># cd /System/Library/StartupItems

sudo mkdir StaticRoutes

sudo chmod 0755 ./StaticRoutes

cd StaticRoutes

touch StaticRoutes && touch StartupParameters.plist

chmod 0644 ./* && chmod o+x StaticRoutes

</pre>

This gets you a directory called StaticRoutes, which contains two files, StaticRoutes and StartupParameters.plist. We've also configured the proper permissions on these files.